1. Field of the Invention
The present invention relates to a display device having a display panel in which capacitive luminescent elements such as electroluminescent elements are used.
2. Description of Related Art
An example of a circuit for driving an electroluminescent display panel is disclosed in U.S. Pat. No. 5,847,516. In this example, an electroluminescent panel having scanning and data electrodes and pixels arranged in a matrix is driven by scanning and data electrode driving circuits. The display panel is sequentially scanned with a positive voltage in a positive field and with a negative voltage in a negative field. Specifically, in the positive field, a scanning voltage Vr is sequentially supplied to the scanning electrodes and an offset voltage that is the same as a modulation voltage Vm is used as a base voltage. A ground voltage Vg is supplied to pixels to be activated and a modulation voltage Vm is supplied to the pixels not to be activated from the data electrodes. In the negative field, a scanning voltage -(Vr-Vm) is sequentially supplied to the scanning electrodes and the ground voltage Vg is used as a base voltage. The modulation voltage Vm is supplied to pixels to be activated and the ground voltage Vg is supplied to the pixels not to be activated from the data electrodes.
The pixels on which voltage Vr is imposed emit light, while the pixels on which voltage (Vr-Vm) is imposed do not emit light. Thus, the pixels arranged in a matrix are selectively activated thereby to display images on the panel. After scanning on the scanning electrodes is completed, electric charges stored in the pixels connected to the scanned electrodes are discharged.
However, there is a following problem in the conventional device disclosed. Since the charges stored in the scanned pixels are discharged when the fields are switched and after the scanning of a selected scanning electrode is completed, a turnaround current is supplied to the scanned pixels when other scanning electrodes are scanned thereafter. That is, since the base voltage of each scanning electrode is set to the voltage Vm that is the same as the modulation voltage in the positive field, the discharged pixels are charged with Vm when the data electrode voltage becomes the ground level voltage Vg (Vg is zero volt). The base voltage of each scanning electrode is set to the ground voltage Vg in the negative field. Therefore, the discharged pixels are charged with the modulation voltage Vm when the data electrode voltage becomes Vm. The turnaround current flowing in this manner does not contribute to luminescence of the pixels, and accordingly this turnaround current is useless and only increases power consumption.
Further, since the turnaround current flows when the data voltage becomes a level to activate the pixels, driving voltage waveforms are deformed or distorted especially when output current for driving the pixels is low, thereby causing uneven brightness among pixels. Such a waveform deformation or distortion is caused by changes of driving voltages and data voltages. For example, when the scanning voltages (Vr and Vm in the positive field, and -(Vr-Vm) and Vg in the negative field) are supplied from a common power source circuit in both fields, the offset voltage Vm is decreased by the turnaround current due to resistance between the scanning electrode driving circuit and the power source circuit in the positive filed, and the ground voltage Vg is increased in the negative field. In accordance with those voltage changes, the voltages Vr and -(Vr-Vm) also change, making the driving voltage insufficient to activate the pixels. Especially, when a scanning electrode having many pixels to be activated is scanned after a series of scanning of electrodes having no pixels to be activated, the amount of the turnaround current becomes large, and thereby the driving voltages Vr and -(Vr-Vm) change in a high degree. In addition to the driving voltage changes, the data voltage waveforms are also changed by the turnaround current, thereby causing the uneven brightness mentioned above. The degree of the uneven brightness becomes especially large when it is required to make a pulse width narrow for a stepwise control of brightness, because a sufficient voltage cannot be imposed on the pixels on a selected scanning electrode.